Why collision detection is NOT enough...

... and why you, imho, ALSO need (rudimentary) line of sight.

let's talk about the tactical purpose of melee characters. They basically build a front line that

• constructs a mobile wall for enemy players
• lures the enemys attention to them
• enables wounded players to back up a little bit to (get) heal(ed)

the later two are only possible when, in addition to collision detection, a line of sight mechanism is implemented.

by collision, you only hinder enemy movement and thus melee characters. to provide opportunities for retreat, you also need to encumber ranged characters. that's what line of sight would be for then: if there's an obstacle b/w the ranged damage dealer / source and the target, the effectiveness of ranged damage sources should be reduced according to the degree of encumbrance. e.g. if a melee character / obstacle is very close to (quite far away from) a ranged damage dealer, his/her damage and/or chance to hit is significantly (slightly) decreased. to simplify, this may only be applied to player-controlled enemy (non-friendly) players.

implementation of line of sight AND collision detection would, imho, open up lots of interesting and highly enjoyable opportunities concerning tactics and overall gameplay:

• searching for weak spots in the enemy lines
  
• setting up traps or "feints"
• flanked attacks and other maneuvers (to break through to the injured / ranged etc.)
• give incentives for melee players to move the front line closer to the enemy artillery :)
• provide a more realistic feel of combat
• etc.

line of sight would only apply to ranged attacks, since melee are already hindered in several ways:

• they have to be within melee / weapon range
  
• they have to overcome additional defense (parry, block) at frontal attacks
• they need to find a way to attack from the side / from behind to unleash their fury... ;)
• ... while, simultaneously, trying to avoid turning their back / side to enemy players

also posted on the WarhammerAlliance forums.